This invention relates to the design, synthesis and application of novel fluorinated boron based compounds which act as anion receptors in non-aqueous battery electrolytes. As a result, the anion receptors of the present invention can be used as additives to enhance the ionic conductivity and cation transference number of non-aqueous electrolytes. More specifically, the family of anion receptors of the present invention includes borane and borate compounds bearing different fluorinated alkyl and aryl groups.
In the past, research has been conducted on the reduction of ion pairing in non-aqueous electrolytes. The design and synthesis of receptor molecules for the selective complexation of ions has been an active area of research in the last two decades. With respect to lithium batteries, ion pairing accounts for the low lithium transference in non-aqueous electrolytes. To decrease ion pairing, researchers have used either solvents of high dielectric constant or added a neutral ligand to coordinate either the cation or the anion. Coordination with either the cation or anion was expected to increase the cation-anion distance of closest approach and thus decrease ion pair formation. For cation coordination in liquid non-aqueous electrolytes, Salomon [1] has reported the use of crown ether 18-crown-6, Matsuda, et al. [2], the use of 12-crown-4, and Schriever, et al. [3], have used cryptands to decrease ion pair formation in polymer electrolytes.
More recently, research has focused on providing neutral compounds to complex anions. These compounds were summarized in a review article written by F. P. Schmidtchen and M. Berger [4]. However, because anion complexation occurs through hydrogen bonding, these neutral compounds cannot be used in lithium batteries. Lee, et al. have utilized aza-ether based compounds as anion receptor molecules. Electron withdrawing groups were used to substitute amine hydrogen atoms in linear aza-ethers, multi-branched aza-ethers, and cyclic aza-crown ethers [5].
Anion coordination is much more important than cation coordination in lithium battery electrolytes because it offers the ability of increasing both conductivity and lithium transference number. Accordingly, there is still a need in the art of lithium batteries for electrolyte additives which can complex anions, yet are stable in lithium batteries. There is also a need in the art of lithium batteries to enhance the conductivity of inexpensive and environmentally friendly inorganic salts such as LiF, LiCl, LiBr and LiI. In addition, there is a need to increase the transference number of the Li.sup.+ ion. In many non-aqueous electrolytes, in particular polymer electrolytes, the transference number of the Li.sup.+ ion is low. This introduces additional polarization losses in batteries and reduces the utilization of the cathode material.
It is therefore, an object of the present invention to provide a new family of compounds which enhances the conductivity of lithium battery electrolytes by complexing with the anion moiety of the salt, and also increases the transference number of the Li.sup.+ ion in electrolytes.
Another object of the present invention is to increase the conductivity of cost effective electrolyte salts such as LiF, LiCl, LiBr and LiI.
Another object of the present invention is to provide improved electrochemical cells by use of electrolyte additives.